In office two months, he has backpedaled on an array of issues, gingerly shifting positions as circumstances dictate while ducking for political cover to avoid undercutting his credibility and authority. That's happened on the Iraq troop withdrawal timeline, on lobbyists in his administration and on money for lawmakers' pet projects.

"Change doesn't happen overnight," Obama said at a town-hall style event in California on Thursday, seeming to acknowledge the difficulty in translating campaign pledges into actual policy. Asked by a campaign volunteer how his supporters can be most effective in helping him bring the sweeping change he promised, Obama said: "Patience."

The event was part of a weeklong media blitz that Obama had hoped would help sell his budget ¿ the foundation of the health care, education and energy changes he promised in the campaign. But his budget message was overshadowed for much of the week by the public furor over $165 million in executive bonuses paid by American International Group Inc. after the insurance giant had received billions in federal bailout funds.

"There was a lot of excitement during the campaign and we were talking about the importance of bringing about change," Obama told the volunteer. "We are moving systematically to bring about change. But change is hard."

It's the same delicate dance each of his predecessors faced in moving from candidate to president, only to find he couldn't stick exactly by his word. Each was hamstrung by his responsibility to the entire nation and to individual constituencies, changes in the foreign and domestic landscapes, and the trappings of the federal government and Washington itself.

Once in the White House, presidents quickly learn they are only one part of the political system, not in charge of it. They discover the trade-offs they must make and the parties they must please to get things done. Inevitably, they find out that it's impossible to follow through completely on their campaign proposals.

For now at least, Obama's deviations have served only to invite occasional cries of hypocrisy from some Republicans and infrequent grumbles of disappointment from some Democrats. He has popularity on his side, and it seems people mostly are chalking up his moves to much-needed flexibility at a difficult time.

But the shifts could take a toll over time if they become a persistent pattern and the public grows weary. His overall job-performance marks could suffer and jeopardize his likely re-election campaign in 2012. People could perceive him as a say-one-thing-do-another politician and the Democratic-controlled Congress could see him as a weak chief executive.

Obama's moves and maneuvering for political cover run the gamut.

He spent most of the campaign promising to bring combat troops home from Iraq 16 months after taking office, though he left himself wiggle room.

After directing his commanders to map out a responsible pullout, President Obama adjusted that timeline to 19 months and said 50,000 troops, about one-third of the current force, would remain.

Then he took office and had to fill thousands of positions. He did allow former lobbyists to join his administration. But he imposed ethics rules barring them from dealing with matters related to their lobbying work or joining agencies that they had lobbied in the previous two years. In several cases, he has made outright exceptions.

Obama the candidate pledged to curb spending directed at lawmakers' pet projects; they're known in Washington as "earmarks." Obama the president signed an "imperfect" $410 billion budget measure that included 8,500 earmarks.

He had little choice. The measure, a holdover from last year, was needed to keep government from shutting down. But to blunt the fallout, Obama outlined guidelines to ensure tighter restraints on the spending and made a new promise: Future earmarks won't become law so easily.

As for politics, Obama campaigned as a new-style leader who chastised partisanship and renounced divisiveness in Washington. But as president, Obama's White House aides wasted little time pouncing on Republicans and mocking conservative commentator Rush Limbaugh as the GOP's leader.

On fiscal matters, Obama the candidate urged Americans to tighten their belts. Once in office and saddled with recession, though, he signed a $787 billion stimulus measure and outlined a $3.6 trillion budget plan that will plunge the nation deeper into the red. But again he paired the proposal with a new promise, to cut the deficit by more than half by the end of his first term.

Franklin D. Roosevelt ran for office promising to balance the budget. But he reversed course when he took over a country in depression and doled out a spending prescription to revive the economy. He made other shifts as well.

The ailing public didn't view him as wishy-washy or politically calculating, but rather as a president who was experimenting in hopes of finding policy to fix the problems. His charm and communication savvy allowed him to get away with it.

Historians agree that seems to be the model Obama is trying to emulate. "I didnt come here to pass on our problems to the next president or the next generation ¿ I came here to solve them," he said Saturday in his radio and Internet address. A charismatic orator, he's trying to govern with a pragmatic posture while projecting a willingness to compromise.

His mantra these days: "We will not let the pursuit of the perfect stand in the way of achievable goals."

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EDITOR'S NOTE ¿ Liz Sidoti covers the White House for The Associated Press and has covered national politics since 2003.

Copyright 2009 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.